Biological Chemistry - Berezov T. T., Korovkin B. F. 1998

Blood
Respiratory Function of Blood
Transport of Carbon Dioxide by Blood from Tissues to the Lungs

In the body of a person at rest (not performing physical work), approximately 180 ml of Carbon dioxide is transported from the Tissues to the Lungs every minute. This value is easy to calculate. If the respiratory quotient is 0.85, then when tissues consume 200 ml of oxygen per minute at rest, about 170 ml of carbon dioxide must be produced (200 × 0.85). In reality, this figure is slightly higher, since The amount of oxygen consumed at rest ranges from 200 to 240 ml per minute.

Overall, over a 24-hour period, approximately 600 L of oxygen enters The Human Body via inhaled air, and 480 L of carbon dioxide (roughly 942.8 g) is released into the environment, which corresponds to 21.4 mol of carbon dioxide.

The body has several mechanisms for transporting CO₂ from tissues to the lungs. Part of it is transported in a physically dissolved state. The solubility of CO₂ in Blood Plasma is 40 times that of oxygen; nevertheless, given the small arteriovenous P_CO2 difference (the partial pressure of CO₂ in venous blood flowing to the lungs via the pulmonary artery is 60 hPa, whereas in arterial blood it is 53.3 hPa), 12–15 ml of CO₂ can be transported in a physically dissolved state at rest, which accounts for 6–7% of the total carbon dioxide transported.

A certain amount of CO₂ can be transported in the carbamino form. It has been shown that CO₂ can bind to Hemoglobin via a carbamino bond to form carbaminohemoglobin (METABOLISM/2.html">THE CONCEPT OF carbon dioxide directly bound to hemoglobin was first proposed by I.M. Sechenov):

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Carbaminohemoglobin is a very unstable compound and dissociates extremely rapidly in the pulmonary capillaries with the release of CO₂.

The amount of the carbamino form is small: in arterial blood, it constitutes 3 vol. %, and in venous blood, 3.8 vol. %*. The carbamino form accounts for The transport of 3 to 10% of all carbon dioxide passing from the tissues into the blood. The bulk of CO₂ is transported through the blood to the lungs in the form of bicarbonate, with erythrocyte hemoglobin playing a crucial role in this process.

As noted, the acidic nature of oxyhemoglobin is significantly more pronounced than that of hemoglobin (the dissociation constant of HHbO₂ is approximately 20 times greater than that of HHb). It is also important to remember that oxyhemoglobin arriving in the tissues with the blood is a stronger acid than H₂СО₃ and is bound to a potassium cation. This potassium salt of oxyhemoglobin can be designated as KHbO₂ (Fig. 17.7). In the peripheral capillaries of the systemic Circulation, erythrocyte hemoglobin releases oxygen to the tissues (KHbO₂ → O₂ + KHb), and its capacity to bind hydrogen ions increases. Simultaneously, the metabolic product carbon dioxide enters the erythrocyte. Under the Influence of the enzyme Carbonic anhydrase **, carbon dioxide interacts with Water to form carbonic acid. The excess hydrogen ions generated by carbonic acid bind to the deoxygenated hemoglobin, while the accumulating HCO₃^- anions pass from the erythrocyte into the plasma ***:

In exchange for these ions, chloride anions—for which The erythrocyte membrane is permeable—enter the erythrocyte, while sodium, the other component of sodium chloride present in the blood, remains in the plasma. As a result, the concentration of sodium bicarbonate (NaHCO₃) increases in the blood plasma.

This process helps restore the alkaline reserve of the blood, meaning that the bicarbonate buffer system is in fairly close functional correlation with the erythrocyte buffer system.

* The content of the carbamino form of CO₂ in venous blood is 1.5–2.0 mmol/L, and in arterial blood, 1.0 mmol/L.

** Carbonic anhydrase exists in several molecular forms (Isoenzymes A, B, and C), which can be separated using Electrophoresis.

*** Arterial blood contains HCO₃^-: 25.5 mmol/L in plasma, 12.7 mmol/L in erythrocytes; in venous blood, the corresponding values are 26.4 and 13.9 mmol/L.

Fig. 17.7. The Role of the plasma-erythrocyte system in the RESPIRATORY FUNCTION OF blood (according to G.E. Vladimirov and N.S. Panteleeva).

a — chemical processes in the pulmonary capillaries; b — chemical processes in the tissue capillaries.

In the pulmonary capillaries, within the erythrocytes, carbonic acid is displaced from potassium bicarbonate by oxyhemoglobin:

HHbO₂ + K⁺ + HCO₃^- → KHbO₂ + H₂CO₃.

The resulting carbonic acid rapidly breaks down into carbon dioxide and water with the participation of carbonic anhydrase. The low P_CO2 in the alveolar lumen promotes the diffusion of carbon dioxide from the erythrocytes into the lungs.

As the bicarbonate concentration in erythrocytes decreases, fresh portions of HCO₃^- ions enter them from the blood plasma, while an equivalent amount of Cl^- ions passes into the plasma. The concentration of sodium bicarbonate in the blood plasma within the pulmonary capillaries drops rapidly, but at the same time, the concentration of sodium chloride in the plasma rises, and free hemoglobin in the erythrocytes is converted into the potassium salt of oxyhemoglobin.

Thus, more than 80% of the total carbon dioxide is transported through the blood to the lungs in the form of bicarbonate with the involvement of erythrocyte hemoglobin.